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<h3><a name="SECTION0022164000000000000000">
20.1.6.4 Coupling Widget Variables</a>
</h3> 
<p>
The current-value setting of some widgets (like text entry widgets)
can be connected directly to application variables by using special
options.  These options are <code>variable</code>, <code>textvariable</code>,
<code>onvalue</code>, <code>offvalue</code>, and <code>value</code>.  This
connection works both ways: if the variable changes for any reason,
the widget it's connected to will be updated to reflect the new value. 

<p>
Unfortunately, in the current implementation of <tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> it is
not possible to hand over an arbitrary Python variable to a widget
through a <code>variable</code> or <code>textvariable</code> option.  The only
kinds of variables for which this works are variables that are
subclassed from a class called Variable, defined in the
<tt class="module"><a href="module-Tkinter.html">Tkinter</a></tt> module.

<p>
There are many useful subclasses of Variable already defined:
<tt class="class">StringVar</tt>, <tt class="class">IntVar</tt>, <tt class="class">DoubleVar</tt>, and
<tt class="class">BooleanVar</tt>.  To read the current value of such a variable,
call the <tt class="method">get()</tt> method on
it, and to change its value you call the <tt class="method">set()</tt> method.  If
you follow this protocol, the widget will always track the value of
the variable, with no further intervention on your part.

<p>
For example: 
<div class="verbatim"><pre>
class App(Frame):
    def __init__(self, master=None):
        Frame.__init__(self, master)
        self.pack()
        
        self.entrythingy = Entry()
        self.entrythingy.pack()
        
        # here is the application variable
        self.contents = StringVar()
        # set it to some value
        self.contents.set("this is a variable")
        # tell the entry widget to watch this variable
        self.entrythingy["textvariable"] = self.contents
        
        # and here we get a callback when the user hits return.
        # we will have the program print out the value of the
        # application variable when the user hits return
        self.entrythingy.bind('&lt;Key-Return&gt;',
                              self.print_contents)

    def print_contents(self, event):
        print "hi. contents of entry is now ----&gt;", \
              self.contents.get()
</pre></div>

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